This invention relates to producing shale oil and related materials from a naturally fractured and leached portion of a subterranean oil shale formation of the type encountered in the Piceance Creek Basin in Colorado.
Numerous portions of subterranean oil shale formations of the above type contain substantially impermeable kerogen-containing minerals mixed with water-soluble minerals or heat-sensitive minerals which can be thermally converted to water-soluble materials. A series of patents typified by the T. N. Beard, M. N. Papadopoulos and R. C. Ueber Pats. 3,739,851; 3,741,306; 3,753,594; 3,759,328 and 3,759,574 describe processes for recovering shale oil from portions of subterranean oil shale formations which are substantially free of interconnected flow paths. However, where an oil shale formation containing such mixtures of components has been naturally fractured and/or leached, the impermeable kerogen-containing components tend to be surrounded by a network of interconnected flow paths. In such a flow path-permeated formation the capture of the shale oil which is generated is difficult unless the path to a nearby production well is the path of least resistance.
The M. J. Tham and P. J. Closmann U.S. Pat. No. 3,880,238 relates to downflowing an oil shale pyrolyzing fluid through a rubble-containing cavern and discloses that plugging can be avoided by keeping the cavern substantially liquid free by using (as a pyrolyzing fluid) a mixture of (a) fluid which is significantly miscible with at least one organic or inorganic solid component of the oil shale or its pyrolysis products, and (b) fluid which is substantially immiscible with such materials. The P. J. Closmann U.S. Pat. No. 4,026,359 relates to producing shale oil from a "leached-zone" subterranean oil shale by conducting a generally horizontal steam drive between injection and production locations in the lower portion of the leached-zone until the production becomes impaired by plugging near the producing location, then injecting steam through that location while producing from a location substantially directly above it. The G. Drinkard U.S. Pat. No. 4,026,360 relates to producing shale oil from a leached-zone subterranean oil shale formation from within a fluid-confining barrier, by (a) reacting the formation components with hot alkaline fluid to form a barrier and (b) conducting an in situ pyrolysis of the oil shale within the confines of the barrier.